1. Field of the Invention
The present invention relates to a method for scanning an examination subject using a CT device with a radiation source with a focus which can be displaced around about a system axis with a radiation beam emanating from the focus and striking a detector system which produces scanning data, and wherein the effective radiation is a first volumetric region, in which a body part of an examiner is located during the scanning, is reduced with respect to the effective radiation in a comparable second volumetric region of the same size and at the same distance from the system axis in which no body part of the examiner is located.
2. Description of the Prior Art
CT devices and known which have a radiation source, for example an X-ray tube, which directs a collimated, pyramidal radiation beam through the examination subject, for example a patient, onto a detector system assembled from a number of detector elements. The radiation source and, depending on the design of the CT device, the detector system as well, are fitted on a gantry which rotates around the examination subject. A support device for the examination subject can be displaced or moved along the system axis relative to the gantry. The starting position from which the radiation beam penetrates the examination subject, and the angle at which the radiation beam penetrates the examination subject, are continuously varied as a consequence of the rotation of the gantry. Each detector element of the detector system, when struck by the radiation, produces a signal which constitutes a measure of the total transparency of the examination subject for the radiation emanating from the radiation source on its path to the detector system. The set of output signals of the detector elements of the detector system, which set is obtained for a specific position of the radiation source, is known as a projection. A scan comprises a set of projections which have been obtained at different positions of the gantry and/or different positions of the support device. The CT device picks up a multiplicity of projections during a scan, in order to be able to build up a two-dimensional tomographic image of a section of the examination subject. A number of sections can be picked up simultaneously with using a detector system formed as an array of a number of rows and columns of detector elements.
Relatively large volumes of the examination subject are usually picked up by means of sequential scanning or spiral scanning. In the case of sequential scanning, the data are picked up during the rotary movement of the gantry, while the examination subject is located in a fixed position, and thus planar sections are scanned. The examination subject is moved between the scanning of successive sections into a new position in which the next section can be scanned. This process continues until all sections designated before the examination are scanned. In the case of spiral scanning, the gantry rotates continuously with the radiation source around the examination subject, while the support table and the gantry are continuously displaced relative to one another along a system axis. Relative to the examination subject, the radiation source therefore describes a spiral path until the volume designated before the examination has been scanned. Images of individual sections are then calculated from the spiral data.
Furthermore, CT devices are known in the case of which the X-ray power can be modulated during the rotation of the radiation source around the examination subject with a non-circular cross section in order to scan the examination subject. If, for example, a patient lying on his or her back is being scanned, as a rule the path of the X-ray radiation through the body of the patient is longer in the horizontal direction than in the vertical direction. If a modulation of the X-ray power is not possible, the power must be set such that the signal quality supplied by the detector system is still sufficient to calculate correct images even for the projections with the longest path of the radiation through the body. The X-ray power therefore is unnecessarily high for all other projections. In order not to stress the object under examination with an excessive radiation dose, an attempt is made to set the X-ray power in accordance with the attenuation profile as a function of the angular position of the radiation source. Such a method is described, for example, in German OS 19 806 063.
CT devices are used principally in the field of medicine. In addition to examinations for purely diagnostic purposes, interventions (for example biopsies, centeses) are increasingly being carried out with the aid of CT monitoring. During the intervention, the position of medical instruments required to carry out the intervention, for example a needle, can thus be monitored continuously. When a radiation source is switched on and manual guidance of such a medical instrument is employed by an examiner, body parts of the examiner, for example a hand, which are located in the region penetrated by the radiation beam between the focus and detector system can be struck by unattenuated radiation.
U.S. Pat. No. 5,873,826 discloses an X-ray CT device wherein the radiant power of the X-ray source can be temporarily reduced during scanning in order to reduce the radiation dose to an examiner. The volumetric region for which this reduction is effective is designated before scanning, and is identified during scanning by marking with a light source.
U.S. Pat. No. 5,841,830 discloses a CT device wherein diagnostic image information is obtained with x-rays at a first intensity, and image information relating to the movement of an invasive surgical instrument is obtained with x-rays at a second intensity, reduced by comparison with the first intensity. The diagnostic image information and the image information relating to the movement of the surgical instrument are superimposed to form a resulting image.
An object of the present invention is to provide a method for operating a CT device such that the radiation dose to an examiner is reduced and, at the same time, a good quality of the calculated images is achieved. It is also an object of the invention to provide a CT device for carrying out the method.
The above object is achieved in accordance with the principles of the present invention an x-ray CT apparatus of the type described above, and a method for operating such a CT apparatus, wherein movement of a body part of an examiner into an examination region covered by the x-ray beam is automatically detected, and the effective radiation is automatically reduced for a first volumetric region which includes the body part of the examiner, with respect to a comparable second volumetric region in which no body part of the examiner is located. The radiation dose to the examiner is thereby reduced. The size of the first volumetric region is automatically adjusted dependent on a detected size and/or position and/or movement direction of the body part. The adjustment of the effective radiation in the volumetric region in which the body part is located can be achieved by varying the tube current of the x-ray tube, by adjusting a beam diaphragm through which the radiation beam passes, or by inserting a radiation absorber into the beam path.
An important advantage of the inventive method and apparatus is that the radiation dose to the examiner is reduced without the examiner needing to define, before starting the scanning, a volumetric region inside the examination space into which the examiner will bring one or more body parts during the examination. The invention thus advantageously simplifies the operation of the CT device, and incorrect inputs are prevented. The invention provides a further advantage by allowing the size of the volumetric region for which the effective radiation is reduced to be limited to a minimum. This is because, for the personal safety of the examiner, the examiner need not establish, before the examination, a relatively large region which he or she must (as far as possible) not exit. The method according to the invention also eliminates the continuous and troublesome monitoring during the examination as to whether this region has actually been exited.
Without limiting the scope of the invention, it is assumed in the further description that the body part of the examiner which is located in the examination space of the CT device is his or her hand.
Various possibilities exist in order automatically to limit the region in which the examiner""s hand is located and for which the radiation intensity is to be reduced. In one version of the invention the position of the hand along the system axis is detected and taken into account when generating the aforementioned parameters. In another version of the invention the angular range of the segment in which the examiner""s hand is located is automatically detected and taken into account when generating the parameters. In this case, the spacing of the hand from the radiation source, and the positions of the focus, the examination subject and the hand relative to one another also can be taken into account. Thus, for example, it is possible with undiminished radiation intensity to obtain X-ray images in the angular range in which the examination subject is situated between the focus and the hand. The radiation intensity is reduced only for the angular region in which the hand is located between the focus and the examination subject. This has the advantage that most of the projections can be obtained without loss of quality during revolution of the radiation source through 360xc2x0 around the examination subject. This therefore involves only a relatively light radiation stress burden for the examiner because in the case of projections where the examination subject is situated between the focus and the examiner, the radiation intensity is already attenuated by the examination subject and the spacing between the focus and the examiner is also relatively large for these projections. Since the radiation dose decreases with the square of the distance from the focus, the final outcome for these projections is only a slight radiation stress for the examiner, while still achieving good quality of the projections. In this version, as well, the region for which the radiation intensity is to be reduced can be established simply and quickly by determining a few parameters. Of course, a combination of the above versions also can be used for the automatic generation of parameters. Thus, the z-position, the angular range and the spacing of the hand from the system axis can be detected jointly in order to generate corresponding parameters.
For the purpose of automatically detecting the position of the examiner""s hand in the examination space of the CT device, the CT device has a suitable detection and evaluation system. Such navigation systems for determining the position and movement of objects are sufficiently known. Their mode of operation can be based on different methods such as optical, magnetic or electromagnetic methods. Of course, it is also possible for this purpose to evaluate the data generated by the CT device itself during scanning.
The automatic dimensioning of the region penetrated by the radiation beam, and in which the examiner""s hand is located, using a detection and evaluation system, has the advantage that such a system can significantly more accurately determine this region and adapt it dynamically during scanning than would be possible if this were done manually by the examiner. The system reacts immediately to movements of the hand in the examination space, and a corresponding adaptation of the region with reduced radiation is performed. Consequently, the extent of this region always can be kept relatively small. A control unit is connected between the computer of the CT device and the radiator assembly (radiation source, radiation diaphragm, etc.) for processing the data generated by the detection and evaluation system and for controlling the radiation intensity, for example, on this basis.
The radiation dose to which the examiner is subjected can be reduced in various ways. In a first embodiment, for this purpose the radiant power of the radiation source is temporarily reduced during rotation around the examination subject. The radiation source is usually an X-ray tube, for which the radiant power can be varied by influencing the tube current. The overall dynamic range of the X-ray tube from zero to a maximum radiation intensity is available in this case for control. Also possible are signal characteristics which change discontinuously or continuously, for example sinusoidal signal characteristics, as well as periodic signal characteristics. Thus, any desired radiation intensity between zero and a maximum value can be set for each z-position and each angular position in the examination space.
The invention provides a further possibility for reducing the radiation intensity by temporarily constricting size of the radiation beam in a plane perpendicular to the radiation propagation direction. This constriction size of the radiation beam reduces the angular range, and thus the period of time, in which the examiner is exposed to the radiation. This also has the effect of diminishing the applied radiation dose. The constriction of the radiation beam can be produced by adjusting a radiation diaphragm at the tube. The constriction can remain the same for a specific z-region, but also can be dynamically adjustable during a revolution of the radiation source around the examination subject. In this case, the radiation beam can be blocked out entirely or partially in a specific angular range and/or z-region. It also is possible to adopt asymmetric diaphragm settings.
In another version of the invention, the radiation intensity in the relevant region is reduced by inserting an absorber between the focus and the hand. It is advantageous in this version for the absorber to remain essentially fixed after being positioned. Once the absorber is correctly positioned, there is a need, as a rule, for only small corrections during scanning. This has a positive effect on the quality of the images produced, since it is possible to avoid the acceleration of relatively large masses during scanning as is necessary, for example, for the dynamic adjustment of diaphragm plates for temporarily constricting the radiation beam. The magnitude of the reduction in the radiation intensity by the absorber can be established by the absorber material and the thickness of the absorber. This magnitude also can have a range from a slight reduction to a complete blocking of the radiation. In addition to an absorber which is fixed after being positioned, it is possible to have an absorber which rotates with the radiation source.
A combination of the above versions for reducing the radiation dose applied to the examiner also can be used. Thus, it is possible during scanning both to constrict the radiation beam, and to lower the radiant power of the X-ray tube, for a specific region.
Reducing the radiation intensity required for carrying out scanning correctly leads as a rule to a loss in quality of the images produced. It is expedient for this loss in quality to be kept as low as possible. The invention offers the possibility of achieving this by obtaining data that are missing from projections with reduced radiation by interpolating data from projections with normal radiation. This can be done effectively, in particular, when the relevant region of the object under examination is small. In another embodiment missing data of projections with reduced radiation at least partially replaced by data from projections which were acquired without the reduction. For this technique, however, it is assumed that the examination subject does not move with respect to the support device. This technique is based on the fact that instruments used for the intervention are very easily visible, with a high contrast in the calculated images, and that movement of such instruments therefore can be followed easily, even in the case of diminished radiation intensity, whereas the examination subject does not change position and the data relating to the examination subject can be used from projection-to-projection.
In a further version of the method according to the invention, missing data of projections with reduced radiation can be calculated from data of complementary projections. In this case, a projection complementary to a given projection means a projection in which the radiation source is offset by 180xc2x0 in the circumferential direction. In the case of radiation intensity being reduced only for a specific angular range, the data calculated therefrom are of high quality as a rule and are therefore well suited for producing images.